VANADIUM IN SEPIOLITE - A REDOX-INDICATOR FOR AN ANCIENT CLOSED BRINESYSTEM IN THE MADRID BASIN, CENTRAL SPAIN

The reconstruction of redox conditions in ancient environments is diff icult in the absence of authigenic Fe and Mn phases as indicators. The Miocene sepiolite beds in the Madrid Basin (central Spain) that were formed in closed systems under arid to semiarid climate are examples o f an environment poor in Fe and Mn phases. In this paper, a new approa ch is presented that uses structure-bound transition metal ions in sep iolites from two localities (Vallecas and Valdemoro) to deduce redox c onditions during their formation. Scanning electron microscopy reveale d that most of the sepiolite is fibrous and that a subordinate part ha s a spheroidal morphology. Electron spin resonance spectroscopy showed Fe(III), V(IV), and V(III) located in octahedral sites of the sepioli te structure. Bound V(III) indicates sulfidic conditions during the cr ystallization of the sepiolite fibers that form the major part of the deposits. The cessation of the formation of sepiolite deposits is indi cated by the poorly crystallized spheroidal morphotype. The occurrence of Mn-oxides in the pore space of the sepiolite deposits indicates la ter oxic conditions that led to a partial oxidation of the metal cente rs in the sepiolite structure.